Hi, (1+) :: Num a => a -> a For the bind operator, you need something of type a -> Maybe b on the RHS, not simply a -> a. You want one of these instead: fmap (1+) (Just 3) liftM (1+) (Just 3) Alternatively, you may find it useful to define something like: (>>*) = flip liftM so that you can write: Just 3 >>* (1+) which bears a closer resemblance to the bind notation. Thanks, Neil. michael rice wrote:
Why doesn't this work?
Michael
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data Maybe a = Nothing | Just a
instance Monad Maybe where return = Just fail = Nothing Nothing >>= f = Nothing (Just x) >>= f = f x
instance MonadPlus Maybe where mzero = Nothing Nothing `mplus` x = x x `mplus` _ = x
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[michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> Just 3 >>= (1+)
<interactive>:1:0: No instance for (Num (Maybe b)) arising from a use of `it' at <interactive>:1:0-14 Possible fix: add an instance declaration for (Num (Maybe b)) In the first argument of `print', namely `it' In a stmt of a 'do' expression: print it Prelude>
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